Gas turbines play a predominant role in a number of applications, namely in aircraft propulsion, marine, propulsion, power generation and driving processes, such as pumps and compressors. Typically, a gas turbine includes a compressor, a combustor and a turbine. In operation, air is fed into the system where it is compressed by a compressor and a portion of the air further mixed with fuel. The compressed air and fuel mixture are then burned to cause an expansion, which is responsible for driving the turbine.
In an effort to reduce emissions, combustors have been designed to premix fuel and air prior to ignition. Premixed fuel and air burn at a lower temperature than the stoichiometric combustion, which occurs during traditional diffusion combustion. As a result, premixed combustion results in lower NOx emissions.
A typical combustor includes a plurality of primary fuel nozzles that surround a central secondary nozzle. Traditional secondary nozzles may include passageways for diffusion fuel and premix fuel all within the same elongated tubular structure. This type of nozzle often includes a complex structure of passageways contained within a single tubular shell. The passageways for creating the diffusion flame extend through the length of the nozzle. Premix fuel is dispensed upstream of the diffusion tip in order to allow fuel to mix with compressed air flowing through the combustor prior to reaching the flame zone, which is located downstream of the nozzle. As a result, passageways for premix fuel are typically shorter than passageways for diffusion fuel.
Additionally, premix fuel may be mixed with air upstream of the diffusion tip and, more importantly, radially outward of the secondary nozzle structure. In this type of secondary nozzle, premix fuel is carried along only a portion of the nozzle length until it is passed radially outward from the nozzle body to a premix injector tip. At the injector tip, the premix fuel is dispensed into the air flow path. As the fuel and air continue to travel downstream along the remainder of the secondary nozzle length, they become mixed, allowing for more efficient combustion within the flame zone, downstream of the nozzle tip.
While compressed air is hot, fuel is typically cool in comparison. The temperature differentials flowing through the different passageways in the secondary nozzle may result in different levels of thermal expansion of the materials used to construct the nozzle. It is contemplated that it would be beneficial to simplify the secondary nozzles to reduce the high stresses on the nozzle structures resulting from their internal complexity, extreme operating conditions and thermal expansion differentials.